The field of the disclosure relates generally to turbine generator systems, and, more specifically, to systems and methods for synchronizing two generators during startup on a turning gear.
At least some known turbine generator systems include a high-pressure (HP) unit and a low-pressure (LP) unit that have their own generators. In known cross-compound turbine generator systems, when a two pole (HP unit) generator is synchronized to a four pole (LP unit) generator on a turning gear, the field voltage for excitation is applied to both generators' rotors simultaneously. However, simultaneous field excitation during synchronization on a turning gear may be problematic because it depends on the location of the north and south poles of both generators when their rotors are excited. During turning gear startup of known cross-compound turbine generator systems, it is difficult, if not impossible, to determine the relative positions of the poles in either generator solely through excitation. When the HP pole leads the LP pole, for example, the LP rotor must increase speed to lock, i.e., “mesh”, with the HP rotor, which may cause the LP rotor to come off of the turning gear as it is accelerated. In addition, during turning gear startup of known cross-compound turbine generator systems, deceleration of the HP rotor to mesh with the LP rotor often causes an increased load on the turning gear motor, which may lead to an overload trip, increased maintenance costs, and/or failure.
A similar outcome may arise in known cross-compound turbine generator systems when larger field voltages are used to cause the LP and HP rotors to pull in step faster. In such situations, when the LP pole leads the HP pole as the excitation field voltage is applied, the LP rotor slows down, which raises the load on the LP turning gear motor, and which may cause the HP unit to unmesh due to its acceleration. If the LP and HP rotors are substantially out of phase, the HP unit may accelerate past its synchronous position and may stop rotating. With too small a field on both generators, the LP rotor will continue to rotate and the HP rotor will remain stopped. With a sufficient field induced on both units, the HP unit will over-accelerate, come to a stop, and then start up again when the LP pole catches up to the corresponding HP pole and is in its synchronous position.
With some known cross-compound turbine generator systems, LP and HP generators on turning gear may be synchronized by increasing both generators to their respective synchronous speeds and then using an auxiliary throttle valve to bring the LP rotor speed up to match the HP rotor speed, at which time the excitation field voltage is applied. Similar to the simultaneous field excitation techniques for synchronization, the speed matching approach using the throttle valve may be problematic. As such, with at least some known cross-compound turbine generator systems, both synchronization approaches may lead to lengthy synchronization times during startup, increased stress and maintenance for turning gears, and require the use of complicated and expensive feedback and control systems. Further, known synchronization approaches for cross-compound turbine generator systems generally increase generation system maintenance costs and outage times.